Key escrow cryptography systems allow authorized parties to retrieve escrowed keys for encrypted communications from escrow and use them to obtain the authorized system user's current session key. An authorized party in possession of the session key can decrypt the user's intercepted communications. Authorized parties are considered to be law enforcement agencies, national security agencies, corporate managers or supervisors and others who at times need access to cryptography communications.
In a public key escrow cryptography system, each user member of the system has a public escrow key/private escrow key pair. The private escrow key member of the pair is divided into parts and the respective parts are deposited with two or more key escrow agents. A user member sends in each communication a law enforcement access field (LEAF) containing the current session key encrypted by the public escrow key corresponding to the combined private escrow key parts deposited with the escrow agents. In a law enforcement environment, a court order (for wiretapping or eavesdropping) is obtained by the law enforcement agency authorizing the key escrow agents to release the private escrow key parts. The law enforcement decryptor combines the key pads to produce the private escrow key and uses it to decrypt the law enforcement access field containing the session key being used in the current communication. With the session key thus obtained, the law enforcement agency can decipher the intercepted communication.
In the public key escrow cryptography system, the receiver (by convention called Bob) of the communication reconstructs the law enforcement access field to verify that the received law enforcement access field in the communication is valid and correct for the current encrypted message. This procedure counters single rogue attacks (an attack on the system by a rogue user), by indicating to the receiver that the communication is from a sender (by convention called Alice) who has included a law enforcement access field in the communication and is properly using the escrow protocol. In order for Bob to reconstruct the law enforcement access field, Alice includes in the communication an escrow verification string (EVS) encrypted with the current session key (the current session key is known to both Alice and Bob). The escrow string incudes Alice's public escrow key and a certificate for Alice's public escrow key in Alice's program. Only an authorized public escrow key should have a certificate, so if the signature verifies, Bob knows that the escrow agents have the corresponding private escrow key. Bob re-encrypts the session key with Alice's public escrow key and compares the result to the received law enforcement access field to see that they are the same.
Bob, or any other member with which Alice communicates can copy, i.e., harvest Alice's public escrow key and the certificate. With the certificate copy, Bob can impersonate Alice, by communicating with others using a LEAF formed with Alice's public escrow key which makes it appear to the law enforcement decryptor that Alice is involved in the communication. The law enforcement decryptor then monitors all of Alice's communications, a situation which causes Alice's communication to be subjected to unwarranted and unauthorized exposure. Bob can also harvest the certificates for may users and variably use certificates among the collection during subsequent communications to confuse law enforcement.
Encrypting the certificate with the session key protects it from eavesdroppers. However, Bob knows the session key and must see the certificate to verify that Alice's purported public escrow key is valid, so Bob automatically has the capability to harvest Alice's and other users' certificates, public escrow key and other information.
The public key escrow cryptography system and the harvesting attack is further described in an article entitled "A New Approach to Software Key Escrow Encryption" by Balenson et al (1994). This article also suggests a solution to the harvesting problem that involves splitting the session key and encrypting each part with one of the respective public keys of the two escrow agents. This procedure still uses a LEAF and an escrow verification string, but does not contain information that can be used by the receiver to impersonate the sender. However, the escrow agents are required to be on-line and involved with every decryption of a new session key.